In industries the use of metal products manufacturing by compaction and sintering metal powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced and the quality requirements are continuously raised at the same time as it is desired to reduce the cost. As net shape components, or near net shape components requiring a minimum of machining in order to reach finished shape, are obtained by pressing and sintering of iron powder compositions, which implies a high degree of material utilisation, this technique has a great advantage over conventional techniques for forming metal parts such as moulding or machining from bar stock or forgings.
One problem connected to the press and sintering method is however that the sintered component contains a certain amount of pores, decreasing the strength of the component. Basically there are two ways to overcome the negative effect on mechanical properties caused by the component porosity: 1) The strength of the sintered component may be increased by introducing alloying elements such as carbon, copper, nickel molybdenum etc. 2) The porosity of the sintered component may be reduced by increasing the compressibility of the powder composition, and/or increasing the compaction pressure for a higher green density, or increasing the shrinkage of the component during sintering. In practise a combination of strengthening the component by addition of alloying elements and minimising the porosity is applied.
There are three common ways of alloying iron powders: prealloying, admixing and diffusion alloying.
During sintering, metal powder particles of the compacted or pressed component, the green component, will diffuse together in solid state forming strong bonds, so called sintering necks. The result is a relatively high dense net shape, or near net shape, part suitable for low or medium performance applications. Typically, sintered articles are manufactured from iron powder mixed with copper and graphite powders. Other types of materials suggested include iron powder prealloyed with nickel and molybdenum and small amounts of manganese to enhance iron hardenability without developing stable oxides. Machinability enhancing agents such as MnS are also commonly added.
As mentioned in US2002/0146341A1, dynamic mechanical properties of a sintered component, such as fatigue strength, are affected by the size of the pores. The lower the amount of large pores present in the sintered structure, the better the dynamic mechanical properties. US2002/0146341A1 describes the use of fine lubricant particles in order to improve dynamic properties.
An effective way to decrease the pore size in a sintered structure is to use finer powders when compacting. However, fine powder compositions are not free flowing and can therefore not be used commercially.
Agglomeration has been suggested to improve flow of fine powders (WO98/25720, U.S. Pat. No. 7,163,569B2), by increasing the mean particle size of the particles in the process. One disadvantage with agglomeration is that porosity will form between the bonded small particles as well as between the agglomerated particles, thus reducing the apparent density of the powder composition and consequently tool cavities with larger fill depth are required.
WO 2007/078232 discloses the use of a combination of fatty alcohol, lubricant and flow agent to reduce powder segregation and dusting, improve powder flow and apparent density, as well as to reduce the ejection force and dimensional spread of the green component of compacted powder. This document does not specifically relate to fine powders.